1. Field of the Invention
The present invention relates to a brushless dc motor having a motor-locked protective circuit. Particularly, the present invention relates to a PWM motor having a motor-locked protective circuit. More particularly, the present invention relates to the PWM motor having a RD (Rotational Detection) signal line to connect between a PWM drive IC member and a PWM converter circuit so that RD signals can maintain the PWM motor to be operated at a predetermined-speed mode when the motor is jammed.
2. Description of the Related Art
Referring to FIG. 1, a conventional brushless dc motor includes a motor drive circuit 1 electrically connected with a motor coil 2 so as to carry out alternatively magnetizing (energizing) the motor coil 2. The alternatively magnetized motor coil 2 can drive a motor rotor (not shown) to turn with respect to a motor stator (not shown) of the motor. Typically, the motor drive circuit 1 includes a PWM drive IC member 10, a Hall IC member 11 and a PWM converter circuit 12. The PWM drive IC member 10 electrically connects with the Hall IC member 11 so as to permit the PWM drive IC member 10 to receive rotor-detecting signals (i.e. rotational detecting signals) generated from the Hall IC member 11. However, the PWM drive IC member 10 is designed to have a pin VTH which electrically connects with the PWM converter circuit 12. Correspondingly, the PWM converter circuit 12 has a PWM input pin 121 serving to introduce a PWM signal from an exterior system (not shown). The PWM signal is converted into a voltage signal by the PWM converter circuit 12, and then sent it to the pin VTH of the drive IC member 10 for controlling or adjusting a period of alternative magnetization of the motor coil 2. Accordingly, operational statuses of the motor are possessed of multi-speed modes in heat-dissipating operation by means of the PWM signal.
Generally, the motor divides the operational statuses into a high-speed mode (including full speed), a low-speed mode (excluding full or zero speed) and a stopping mode (zero speed). The PWM drive IC member 10 can determine the operational statuses of the motor according to the input PWM signal such that the motor can be adjusted and changed in speeds to fulfill various system needs. For example, when a voltage of the pin VTH of the PWM drive IC member 10 is higher than 3.6 volts, the PWM drive IC member 10 controls the motor to operate at the stopping mode as well as zero rpm. Conversely, when the voltage of the pin VTH of the PWM drive IC member 10 is lower than 2.0 volts, the PWM drive IC member 10 controls the motor to operate at the high-speed mode as well as 6,000 rpm. If the voltage of the pin VTH of the PWM drive IC member 10 is in the range of 2.0 volts to 3.6 volts, the PWM drive IC member 10 controls the motor to operate at the low-speed mode as well as greater than zero rpm but lesser than 6,000 rpm.
Referring again to FIG. 1, the motor drive circuit 1 is designed to have a capacitor 3 parallel-connected between the PWM drive IC member 10 and the PWM converter circuit 12. Meanwhile, the capacitor 3 is designed to have a ground connection in place. In operation, the capacitor 3 is adapted to commutate a saw tooth wave input from the PWM converter circuit 12. However, the capacitor 3 of the motor drive circuit 1 is so configured to stabilize the voltage of the pin VTH of the PWM drive IC member 10. When the motor is actuated, the voltage of the pin VTH of the PWM drive IC member 10 can determine and adjust the speed of the motor.
Referring to FIGS. 2A and 2B, the PWM drive IC member 10 can control the motor to operate in the high-speed mode or the low-speed mode. In normal operation, the speed of the motor is operating at 2,000 rpm as well as low-speed mode when the voltage of the pin VTH of the PWM drive IC member 10 is maintained at 3.0 volts (i.e. lesser than 3.6 volts but greater than 2.0 volts). But, in abnormal (high temperature) operation, the speed of the motor is operating at high-speed mode when the voltage of the pin VTH of the PWM drive IC member 10 is dropped to zero volts (i.e. lesser than 2.0 volts).
Still referring to FIGS. 2A and 2B, due to a ground connection, the voltage across the capacitor 3 is generally zero volts, as best shown in FIG. 2A, and the capacitor 3 can be charged by a voltage from a power supply when the motor is started. Inevitably, the voltage of the pin VTH of the PWM drive IC member 10 is maintained at substantially zero volts. In this way, the PWM drive IC member 10 can invariably control the motor to operate in the high-speed mode as long as the motor is started; namely, the speed of the motor is rapidly and shortly jumped to 6,000 rpm (i.e. full speed) from zero rpm, as best shown in FIG. 2B.
Referring back to FIGS. 1 and 2B, once started, the motor must inevitably enter the high-speed mode that must rapidly and shortly increase the speed of the motor. However, there is no greater amount of operational heat for dissipation. This results in the motor unnecessarily operating at full speed (i.e. top speed) that generates an increased amount of air noise and vibration. Furthermore, the motor occurs an increased amount of abrasion among motor components that may shorten the longevity of the motor.
Referring again to FIGS. 2A and 2B, the voltage across the capacitor 3 can reach 3.0 volts in the event after charging for a predetermined time. In this way, the voltage of the pin VTH of the PWM drive IC member 10 is greater than 2.0 volts but lesser than 3.6 volts so that the PWM drive IC member 10 terminates the motor to operate in the high-speed mode. Accordingly, the speed of the motor is dropped to a predetermined speed or a lower speed of 2,000 rpm.
However, ambient heat generated from a heat source is lower than a high temperature when the motor is started. Therefore, it is undesirable to permit the PWM drive IC member 10 to increase the speed of the motor reaching 6,000 rpm in the high-speed mode that is unsuitable for the need of normal usage or an improper usage of the motor due to a waste of power consumption. Hence, there is a need for improving the motor to prevent entering the high-speed mode while starting.
In order to solve the motor to be unexpectedly operated at the high-speed mode while starting, an approach to this problem is disclosed in applicant's own U.S. patent application Ser. No. 11/247,417, the entire disclosure of which is incorporated herein by reference. In this approach, a capacitor is parallel connected between a pin VTH of the PWM drive IC member and PWM converter circuit, and the capacitor has an end further connecting with a power source. Accordingly, the voltage of the pin VTH of the PWM drive IC member 10 cannot drop to zero voltage in such a way as to prevent the motor from unexpectedly entering a high-speed mode while starting the motor.
In addition, with reference to FIG. 1, when the motor is normally rotated, the PWM drive IC member 10 can be situated at a normally operational temperature. For instance, if the motor is applied by a rated current of 0.543 amperes, the operational temperature of the PWM drive IC member 10 produces about 36 degrees centigrade that may not damage the PWM drive IC member 10 and other motor electronic components, as is demonstrated in observed experimental results in the study. Conversely, when the motor is jammed (locked) or failed, the PWM drive IC member 10 is successively supplied by the rated current which is suitable for the high-speed mode such that the operational temperature generated from the PWM drive IC member 10 rapidly raises. Even the rated current of the motor may synchronously increase. On the other hand, the normally rated current of the motor is still applied by 0.543 amperes. For instance, if the motor is jammed (locked) or failed, the operational temperature generated from the PWM drive IC member 10 can be rapidly jumped up to 73.1 degrees centigrade. Even the rated current of the motor is increasing about 1.086 amperes, as is demonstrated in observed experimental results in the study. Inevitably, such a high temperature may damage the PWM drive IC member 10 and other motor electronic components.
As is described in greater detail below, the present invention intends to provide a brushless dc motor having a motor-locked protective circuit, wherein a RD (Rotational Detection) signal line connects between a PWM drive IC member and a PWM converter circuit so that RD signals can build a predetermined voltage to control the PWM drive IC. The predetermined voltage can maintain the brushless dc motor to be operated at a predetermined-speed mode when the motor is jammed. In this manner, an operational temperature of the PWM drive IC member cannot be rapidly increased in such a way as to mitigate and overcome the above problem.